This invention relates to a semiconductor light emitting device, in particular to the semiconductor light emitting device made from a nitride semiconductor such as GaN, AlGaN, InGaN, InGaAlN and BInGaAlN which has a lower operating voltage compared to the conventional ones.
Light emitting devices such as a light emitting diode (LED) and a laser diode (LD) which are made of nitride semiconductor are expected to have a high luminous efficiency in a short wavelength region, since it is predicted that a nitride semiconductor such as BInGaAlN is expected to have a band structure of a direct transition-type over almost all the composition range.
It should be noted that the term xe2x80x9cnitride semiconductorxe2x80x9d as used in this application comprises semiconductors of compounds of groups III to V with the generic form of BxInyAlzGa(l-x-y-z)N (where 0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, and 0xe2x89xa6zxe2x89xa61), and further comprises mixed crystals that include phosphorous (P) and/or arsenic (As) in addition to nitrogen (N), as group-V elements.
The composition range is good if the subscripts x, y, z in the formula are within the range mentioned above. For example, GaN doped with an extremely small amount of indium is a xe2x80x9cnitride semiconductorxe2x80x9d according to the specification and in the claims.
As for the light emitting device of nitride semiconductor, there are many cases to use the cladding layer of AlyGa1xe2x88x92yN (0 less than y less than 0.5) for the active layer in order to suppress overflow of a carrier from the active layer made of InGaN or GaN. Aluminum (Al) composition of around 10% is necessary at least in order to suppress the overflow of a carrier effectively. However, when Al composition is increased, it becomes difficult to get ohmic contact with a p-side electrode, thus a problem that operating voltage is increased occurs.
A method to form the p-type contact layer made of p-type InGaN or p-type GaN doped with Mg on the AlGaN cladding layer is mentioned in Patent Laid-Open No. H6-268259 issue bulletin and Patent Laid-Open No. H9-289351 issue in order to reduce operating voltage.
FIG. 8 is the outline sectional view which exemplifies the LED which possess such contact layer.
In the LED shown in the figure, buffer layer 112, n-type GaN layer 113, active layer 114, p-type AlGaN cladding layer 115, p-type GaN contact layer 116 are formed in this sequence on the sapphire substrate 111.
On n-type GaN layer 113, n-side electrode 163 and bonding pad 164 are formed, and on p-type GaN contact layer 116, p-side electrode 165 and bonding pad 166 are formed. In addition, the surface of the device is coated with protective films 161, 162,
According to the structure shown in the figure, by forming the electrode 165 in contact with the p-type GaN contact layer, an ohmic contact is made and the operating voltage can be lowered to some extent.
On the other hand, a trial to reduce the contact resistance with the electrode by increasing the concentration of the magnesium (Mg) in the surface region of the p-type contact layer is described in the Japanese Patent No. 2666237.
However, by these both methods, the operating voltage of the light emitting device is about 3.5V, which is still high.
Another method to lower the operating voltage of an device is decribed in a Patent Laid-Open No. H9-289351, which uses a p-type InGaN contact layer doped with magnesium (Mg). That is, a potential barrier of valence band can be lowered by using InGaN instead of GaN, thus the operating voltage can be reduced. However, actually, as for InGaN, there is a problem to be hard to make the p-type conductivity compared to GaN. Therefore the light emitting device which used this method has not been put to practical use at all.
The present invention has beeen made substrated on recognition of above-mentioned problems. In other words the object of the invention is to reduce the operating voltage, and to offer the nitride semiconductor light emitting device which can work stably by the commonly-used voltage power source.
According to the invention, there is provided a semiconductor light emitting device comprising: a contact layer formed of a nitride semiconductor; and a p-side electrode provided in contact with a surface of the contact layer, the contact layer having an alternatively stacked structure of first nitride semiconductor layers having a wider bandgap and second nitride semiconductor layers having a narrower bandgap, the first semiconductor layers being selectively doped with a p-type dopant.
According to the invention there is provided another semiconductor light emitting device comprising: a contact layer formed of a nitride semiconductcr; and a p-side electrode provided in contact with a surface of the contact layer, the contact layer having an alternatively stacked structure of first nitride semiconductor layers having a higher hardness and second nitride semiconductor layers having a lower hardness, the first semiconductor layers being selectively doped with a p-type dopant.
According to the invention there is provided yet another semiconductor light emitting device comprising: a contact layer formed of a nitride semiconductor; and a p-side electrode provided in contact with a surface of the contact layer, the contact layer having an alternatively stacked structure of first nitride semiconductor layers and second nitride semiconductor layers, the second nitride semiconductor layers including a higher content of indium than the first nitride semiconductor layers.
Generally, doping in the superlattice structure is made preferentially into the layers having a narrower bandgap. However, in a case of a nitride semiconductor, the hardness becomes higher as Al (aluminum) composition rises and bandgap thereof becomes wider, while the hardness becomes lower as In (indium) composition rises and bandgap thereof becomes narrower. Therefore, in this specific material system, it is effective to selectively dope the layers having a wider bandgap and a higher hardness.
The layers having a lower hardness have a function as buffer layers and improve the crystal quality by accommodating the crystal strain. According to the invention, non-doping to these soft layers prevents the degradation of crystal and keeps the function as buffer layers.
According to the invention, the operating voltage of semiconductor light emitting device consisting of a nitride semiconductor can be decreased effectively and the luminescence property can be improved.
In other words, the generation of heat in a contact part is restrained by decreasing the contact resistance at the p-side electrode, and thus the temperature characteristic of the device is improved, and life can also be improved.
In particular, in case of laser diodes, the generation of heat in a contact part with an electrode becomes more serious since a larger amount of current should be injected. According to the invention, the contact resistance at p-side can be advantageously decreased. Accordingly, the operating voltage is decreased and the heat generation is suppressed. As a result, the temperature characteristics is improved and the reliability such as device life is drastically improved.
In addition, it becomes possible to easily operate the semiconductor light emitting device by using a power supply of general-purpose batteries because the operating voltage of the device is decreased. For example, according to the present invention, operating voltage of light emitting device can be decreased to less than 3V. As a result, the light emitting device according to the invention can be used in various kinds of battery-operated machines such as laser pointers, portable computers, personal data assistants and video players.
In other words portable machines having full color display and multicolored display of every kind become possible to be realized, and a merit on industry is great.